Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses. Examples of fluoroelastomers include copolymers comprising units of vinylidene fluoride (VF2) and units of at least one other copolymerizable fluorine-containing monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a fluorovinyl ether such as a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether).
In order to fully develop physical properties such as tensile strength, elongation, and compression set, elastomers must be cured, i.e. vulcanized or crosslinked. In the case of fluoroelastomers, this is generally accomplished by mixing uncured polymer (i.e. fluoroelastomer gum) with a polyfunctional curing agent and heating the resultant mixture, thereby promoting chemical reaction of the curing agent with active sites along the polymer backbone or side chains. Interchain linkages produced as a result of these chemical reactions cause formation of a crosslinked polymer composition having a three-dimensional network structure. Commonly employed curing agents for fluoroelastomers include difunctional nucleophilic reactants, such as polyhydroxy compounds.
However, polyhydroxy cured fluoroelastomer articles may exhibit unacceptably high volume swell, e.g. 50-200 vol. %, that can lead to seal failure, when seals are exposed to biodiesel fuel for long periods of time or at elevated temperatures, especially when the fuel contains a minor amount of water. Biodiesel fuels often contain water as an impurity. The source of the water may be a washing step in the fuel manufacturing process or exposure to moist air during storage. Typical specifications for manufactured biodiesel allow for some water impurity, e.g. ASTM D6751.